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Mn-Andalusite, Spessartine, Mn-Grossular, Piemontite and Mn-Zoisite/Clinozoisite from Trikorfo, Thassos Island, Greece

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Mn-Andalusite, Spessartine, Mn-Grossular, Piemontite and Mn-Zoisite/Clinozoisite from Trikorfo, Thassos Island, Greece Δελτίο της Ελληνικής Γεωλογικής Εταιρίας, τόμος L, σελ. 2068-2078 Bulletin of the Geological Society of Greece, vol. L, 2068-2078 Πρακτικά 14ου Διεθνούς Συνεδρίου, Θεσσαλονίκη, Μάιος 2016 Proceedings of the 14th International Congress, Thessaloniki, May 2016 MN-ANDALUSITE, SPESSARTINE, MN-GROSSULAR, PIEMONTITE AND MN-ZOISITE/CLINOZOISITE FROM TRIKORFO, THASSOS ISLAND, GREECE Voudouris P.1, Graham I.2, Mavrogonatos K.1, Su S.2, Papavasiliou K.3, Farmaki M.-V.1 and Panagiotidis P.1 1Department of Mineralogy and Petrology, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, [email protected], [email protected],[email protected], [email protected] 2School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052 Australia, [email protected], [email protected] 3Department of Economic Geology & Geochemistry, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, [email protected] Abstract Mylonitized manganiferous schists and calc-silicate layers intercalated within amphibolite- to greenschist facies mica schists from the Trikorfo area (Thassos Island, Greece), host an unusual Mn-rich paragenesis of metamorphic silicate minerals, most of them in large, gemmy crystals. The silicates occur both in layers subparallel to the foliation and within discordant veins cross-cutting the metamorphic fabric. Piemontite (up to 12.7 wt. % Mn2O3), Mn-rich epidote (up to 7.8 wt. % Mn2O3), Mn-rich andalusite (up to 15.6 wt. % Mn2O3), Mn-poor pink clinozoisite-epidote (up to 0.87 wt. % Mn2O3), Mn-poor pink zoisite (up to 0.21 wt. % Mn2O3), spessartine (up to 47.7 wt. % MnO) and Mn-rich grossular (up to 3.6 wt. % MnO) are associated with diopside, hornblende, phlogopite, muscovite, tourmaline, hematite and iron-bearing kyanite. The studied assemblages are indicative of high fO2 conditions due to the presence of highly-oxidized pre-metamorphic Mn-rich mineral associations. They developed during prograde metamorphism of a Mn-rich sedimentary protolith(s), followed by re-equilibration to post-peak metamorphic conditions, vein formation and metasomatism during retrograde metamorphism accompanying the exhumation of the Thassos Island during the Oligocene-Miocene. Alternatively, the skarn-similar mineralogy of the calc-silicate layers could have been formed by fluids released by granitoids during contact metamorphism. The studied area represents a unique mineralogical geotope. Its geological-mineralogical heritage should be protected through establishment of a mineralogical-petrological geopark that will also promote sustainable development of the area. Keywords: Mn-andalusite (viridine), Mn-zoisite (thulite), spessartine, geotope, Thassos Island. Περίληψη Μυλωνιτιωμένοι πλούσιοι σε μαγγάνιο σχιστόλιθοι και ασβεστοπυριτικά στρώματα που απαντούν ως ενδιαστρώσεις εντός αμφιβολιτικής έως πρασινοσχιστολιθικής φάσης μεταμόρφωσης μαρμαρυγιακούς σχιστόλιθους στο Τρίκορφο της Θάσου, 2068 χαρακτηρίζονται από μία ασυνήθιστη Mn-ούχο παραγένεση μεταμορφικών ορυκτών τα περισσότερα από τα οποία σε ιδιαίτερα μεγάλους κρυστάλλους και σε ποικιλία πολύτιμων λίθων. Τα Mn-ούχα πυριτικά ορυκτά απαντούν τόσο σε στρώσεις παράλληλα με την φύλλωση όσο και σε φλέβες που τέμνουν τη μεταμορφική δομή. Τα ορυκτά πιεμοντίτης (έως 12.7 % κ.β. Mn2O3), Mn-ούχο επίδοτο (έως 7.8 % κ.β. Mn2O3), Mn-ούχος ανδαλουσίτης (έως 15.6 % κ.β. Mn2O3), φτωχός σε Mn ροζ κλινοζοϊσίτης/επίδοτο (έως 0.87 % κ.β. Mn2O3), φτωχός σε Mn ροζ έως κόκκινος ζοϊσίτης (έως 0.21 % κ.β. Mn2O3), σπεσσαρτίνης (έως 47.7 % κ.β. MnO) και Mn-ούχος γροσσουλάριος (έως 3.6 % κ.β. MnO), συνοδεύονται από διοψίδιο, κεροστίλβη, φλογοπίτη, μοσχοβίτη, τουρμαλίνη, αιματίτη και σιδηρούχο κυανίτη. Η παραγένεση που μελετήθηκε είναι ενδεικτική υψηλών τιμών πτητικότητας του οξυγόνου λόγω της παρουσίας προϋπάρχουσας έντονα οξειδωτικής προ-μεταμορφικής ορυκτολογικής παραγένεσης πλούσιας σε μαγγάνιο. Σχηματίσθηκε κατά την πρόδρομη μεταμόρφωση ιζηματογενών πρωτόλιθων πλούσιων σε Mn, με ακόλουθη επανισσορόπηση απο τις μέγιστες συνθήκες πίεσης και θερμοκρασίας, σχηματισμό φλεβών και μετασωμάτωσης κατά την ανάδρομη μεταμόρφωση που συνόδευσε την ανάδυση της Θάσου στο Ολιγόκαινο-Μειόκαινο. Εναλλακτικά, η προσφορά ρευστών από γρανιτοειδή κατά την διάρκεια μεταμόρφωσης επαφής δεν πρέπει να αποκλειστεί. Η περιοχή μελέτης αντιπροσωπεύει μοναδικό ορυκτολογικό Γεώτοπο. Η γεωλογική-ορυκτολογική αυτή κληρονομιά μπορεί να προστατευθεί μέσω της ίδρυσης ενός Γεωπάρκου που θα συμβάλλει επιπλέον και στην προώθηση φιλικής προς το περιβάλλον ανάπτυξης της Θάσου. Λέξεις κλειδιά: Mn-ανδαλουσίτης (βιριδίνης), Mn-ζοϊσίτης (θουλίτης), σπεσσαρτίνης, γεώτοπος, Θάσος. 1. Introduction Greece hosts several sites containing manganiferous minerals that formed in metamorphic environments, such as the Mn-rich metasediments of Andros and Evia islands (Reinecke, 1986), the high P/T Mn-bearing meta-conglomerates from northern Syros Island (Altherr et al., 2013) and the Mn-bearing meta-skarn on Paros Island (Paraskevopoulos, 1958). The Trikorfo area, Thassos Island, is characterized by an unusual manganiferous mineralogical assemblage hosted within metamorphic rocks of the Rhodope Massif, first described by Zachos (1982), and consist of piemontite, braunite, spessartine, dravite, thulite, epidote, kyanite, staurolite, titanite and hematite. Dimitriadis (1989) reported on the presence of pink pleochroic manganoan epidote in mica schists, as well as, biotite- muscovite-kyanite-sillimanite bearing schists, thus limiting peak metamorphic conditions to 600- 650 °C at 4-7 kbar. Voudouris (2005) and Voudouris et al. (2006, 2012, 2013), described in some detail the mineralogy and mineral-chemistry of the above manganiferous minerals and other silicates from Trikorfo (kyanite, tremolite, tourmaline, chlorite, etc) and reported on the discovery of Mn- rich andalusite ((better known as viridine) in the area occurring within piemontite-spessartine schists and quartz veins as deep emerald-green-colored crystals up to 7cm. The Mn-andalusite, spessartine and thulite from Trikorfo are of world class museum quality, and can be considered as potential gemstones (Voudouris et al., 2012). The aim of the current study is to present new data on the manganiferous silicate association(s) of Trikorfo, to describe the mineralogical and mineral- chemical characteristics and discuss the role of various mechanisms in their formation through comparison it with other similar assemblages from elsewhere. 2. Geology Thassos Island consists mainly of Permo-Carboniferous orthogneisses and marbles intercalated with amphibolites and metapelites interpreted as members of the Southern Rhodope Core Complex, and display an intense, penetrative top-to-the-southwest shearing under amphibolite and greenschist facies conditions (Brun and Sokoutis, 2007). The island comprises three dominant rock units from base to top, separated by two SW-dipping low-angle normal fault systems (Wawrzenitz and Krohe, 2069 1998; Brun and Sokoutis, 2007): (1) a lower unit, consisting mainly of marbles with intercalations of paragneisses and micaschists; (2) an intermediate unit consisting of orthogneisses, marbles with intercalations of paragneisses, mica-schists and amphibolites and (3) an upper unit, dominated by migmatites and gneisses. Two major tectonic events affected the above mentioned lithologies: a compressional phase (of pre-Mid-Eocene age) resulted in the thrusting of the gneisses and migmatites on top of the intermediate unit and an extensional ductile to brittle event that took place from 26-24 to 12 Ma, creating a NE-trending stretching lineation, foliated fabric and widespread boudinage formation (Wawrzenitz and Krohe, 1998; Brun and Sokoutis, 2007; Kounov et al., 2015). Rb-Sr and U-Pb dating by the above authors revealed a continuous cooling history of the ductile metamorphic rocks from 700°C to 300°C from 26 to 12 Ma. Figure 1 - (a) Panoramic view of the Trikorfo area, Thassos Island. (b) Blue kyanite crystals from quartz-feldspar lenses within mica schists. (c) Orange kyanite in quartz lense. (d) Outcrop of piemontite (purple) and Mn-andalusite (green) bearing schists. (e) Mn-andalusite crystals in quartz-feldspar lense. (f) Spessartine from piemontite-spessartine assemblage. (g) Gemmy spessartine with muscovite in quartz. (h,i) Mn-zoisite crystals with grossular and quartz from calc-silicate layers hosted within the mica schists. The study area at Trikorfo is located along the major SW-dipping low-angle normal detachment zone (e.g. reactivated thrust) separating the lower from the intermediate unit at Thasos (Fig. 1a). Mica schists and orthogneisses predominate and tectonically overlay the marbles of Theologos (e.g. lower unit; Fig. 1a). Kyanite-sillimanite micaschists which are close to the main shear zone have formed in P-T conditions of 5.5±1.5 kbar and 600±50°C and bear signs of incipient partial melting contemporaneous with the ductile fabric that is related to extensional deformation (Brun and Sokoutis, 2007). Quartz lenses within the mica schists mostly contain blue kyanite crystals, but in some cases green, yellow and orange kyanite are present (Fig. 1b, c), along with uncommon zoned kyanite crystals with dark blue cores and yellow-green rims. Piemontite-andalusite-muscovite, piemontite-spessartine-braunite- 2070 and kyanite-spessartine-muscovite schists are intercalated within the kyanite-bearing mica schists (Fig. 1d). Quartz- and quartzofeldspathic lenses inside the schists
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